1. Field of the Invention
The present invention relates to power converters, and particularly to a power converter capable of outputting a plurality of different levels of voltages.
2. Description of the Background Art
A power converter that converts direct current (DC) power to alternating current (AC) power by varying continuous output of DC voltages from a plurality of DC power sources during a single cycle has been proposed. This power converter converts DC power to AC power by continuously outputting a plurality of DC voltages of different potentials, rather than generating a constant pulsed voltage like an inverter having a single DC power source. Accordingly, this power converter can continuously output the plurality of DC voltages of different potentials finely without waste, to convert DC power to AC power with suppressed harmonics compared with a power converter having a single DC power source.
For example, Japanese Patent Laying-Open No. 2000-341964 discloses a multilevel inverter as the above-described power converter. According to this patent publication, the multilevel inverter includes redox flow type secondary batteries connected in series and producing multilevel terminal voltages, and an inverter unit for controlling continuous output of potentials of the multilevel terminals to produce AC power. The inverter unit includes a total of eight switching elements and six diodes, and controls the opening/closing of the switching elements in response to instructions from a control unit.
FIG. 7 is a circuit diagram illustrating a circuit configuration of a conventional power converter such as disclosed in the aforementioned patent publication. A power converter 100 illustrated in FIG. 7 is a five-level inverter capable of outputting five different levels of voltages. Power converter 100 includes four DC power sources V, eight switch elements S101 to S108, and six diodes D101 to D106.
Power converter 100 has a midpoint V0 as the middle point between four DC power sources V, midpoint V0 having a voltage level of “0V”. Accordingly, in power converter 100, the first DC power source V on the positive potential side relative to midpoint V0 has a voltage level of “+1V”, and the second DC power source V on the positive potential side relative to midpoint V0 has a voltage level of “+2V”. Conversely, in power converter 100, the first DC power source V on the negative potential side relative to midpoint V0 has a voltage level of “−1V”, and the second DC power source V on the negative potential side relative to midpoint V0 has a voltage level of “−2V”.
Power converter 100 can output a potential having a voltage level of “+2V” from an output terminal by turning switch elements S101, S102, S103 and S104 on, and can output a potential having a voltage level of “+1V” from the output terminal by turning switch elements S102, S103, S104 and S105 on. Power converter 100 can also output a potential having a voltage level of “0V” from the output terminal by turning switch elements S103, S104, S105 and S106 on. Power converter 100 can further output a potential having a voltage level of “−1V” from the output terminal by turning switch elements S104, S105, S106 and S107 on, and can output a potential having a voltage level of “−2V” from the output terminal by turning switch elements S105, S106, S107 and S108 on. Thus, power converter 100 can output five different of levels of voltages (“−2V”, “−1V”, “0V”, “+1V”, “+2V”) from the output terminal.
In power converter 100, however, when switch elements S105, S106, S107 and S108 are turned on in order to output a potential having a voltage level of “−2V” from the output terminal, diodes D102, D104 and D106 each have a voltage level of “−2V” at its anode terminal, with diode D102 having a cathode terminal connected to a voltage level of “+1V”. Therefore, a voltage corresponding to the sum of voltages of three DC power sources V is applied to diode D102. Similarly, a voltage corresponding to the sum of voltages of two DC power sources V is applied to diode D104, and a voltage corresponding to a voltage of one DC power source V is applied to diode D106.
Moreover, in power converter 100, when switch elements S101, S102, S103 and S104 are turned on in order to output a potential having a voltage level of “+2V” from the output terminal, diodes D101, D103 and D105 each have a voltage level of “+2V” at its cathode terminal, with diode D105 having an anode terminal connected to a voltage level of “−1V”. Therefore, a voltage corresponding to the sum of voltages of three DC power sources V is applied to diode D105. Similarly, a voltage corresponding to the sum of voltages of two DC power sources V is applied to diode D103, and a voltage corresponding to a voltage of one DC power source V is applied to diode D101.
As such, in the multilevel inverter disclosed in the aforementioned patent publication, diodes D102 and D105 connecting the DC power sources to the switch elements are required to have a breakdown voltage three times higher than that of diodes D101 and D106, and diodes D103 and D104 are required to have a breakdown voltage two times higher than that of diodes D101 and D106, respectively. For this reason, the multilevel inverter disclosed in the aforementioned patent publication needs to employ diodes having different breakdown voltages, or to connect two or three diodes in series to increase the breakdown voltage, thus increasing the complexity of the apparatus and the difficulty in manufacturing the apparatus.
Furthermore, in the multilevel inverter disclosed in the aforementioned patent publication, increasing the number of levels of voltages to be output requires higher breakdown voltages of the diodes. This increases the complexity of the configuration of the diodes connected between the DC power sources and the switch elements, and further increases the difficulty in manufacturing the apparatus.